The present invention generally relates to a telephone line interface circuit. More particularly, the present invention relates to measuring a characteristic of a telephone line.
It is well know in telephony that the status of a telephone line may be determined by monitoring the voltage present between the T and R leads of the telephone line. For example, when a telephone connected to a telephone line is picked up (i.e., off hook), the line voltage will typically drop substantially. However, as more and more extension telephones connected to the same telephone line are picked up, the line voltage will drop only incrementally, beyond the initial substantial drop associated with picking up the first telephone.
The present inventor recognizes the importance of accurately measuring the telephone line voltage in order to provide the correct status of the telephone line. It is especially important to be able to precisely measure a small incremental voltage drop when an extension telephone phone is lifted off hook, in order to effectively implement, for example, a xe2x80x9cremote hold releasexe2x80x9d feature in a KUS-less (Key Service Unit-less; i.e., system without a key service unit), multi-line telephone system 10 shown in FIG. 1. The remote hold release feature is the capability of a telephone in such a system to drop the telephone""s connection automatically, when the telephone detects that the same telephone line is being picked up via a remote handset.
As an example, to use this feature, a user may first answer a telephone call from line 1 (as indicated by wire pair T1 and R1 in FIG. 1) using a handset 1. In some instances, however, the user may have to go to another desk, where handset 2 may be located, for example, to look up some other information. The user will then place handset 1 on hold and walk to the location of handset 2 to pick up line 1 again. Once handset 1 detects that handset 2 has been used to pick up line 1 at the remote location, handset 1 will automatically release itself from line 1. Therefore, an accurate line measurement circuit is needed to effectively determine when additional telephones are lifted off hook in order to implement the remote hold release feature as described above.
The present inventor also recognizes the problem of designing a telephone line measurement circuit to accommodate different polarities of the voltage present on a telephone line from one central office system to another. That is, on some systems, the potential on the T lead may be more positive than on the R lead, and vice versa. In addition, a person may inadvertently reverse the T and R leads during an installation process.
In addition, the present inventor recognizes the importance of providing a high impedance and high resolution measurement circuit which is capable of detecting only a small voltage change on the telephone line. This is particularly important, as described above, for a multi-line telephone systems to implement various features.
Therefore, a telephone line measurement circuit for detecting a variation of a characteristic between a first and a second leads of a telephone line is described, comprising:
a first circuit having a first input and a second input and an output;
a second circuit, identical to said first circuit, having a first input and a second input and an output; wherein
said first input of said first circuit and said second input of said second circuit are coupled to said first lead of said telephone line and said second input of said first circuit and said first input of said second circuit are coupled to said second lead of said telephone line, and said outputs are coupled together.